DIATOMACEJE. 231 



out a true raplie ; without central and marginal nodules ; without 

 teeth, processes, or spines. 



Family 6. Fragilarieee. Frustules adherent, forming a ribbon- 

 like, fan-like, or zigzag filament, or attached by a gelatinous cushion 

 or stipe ; sometimes arcuate in front, or side view. 



Epithemia, Eunolia, Fragilaria, Synedra, Diatomi. 



Family 7. Tabellariese. Frustules with internal plates, or imper- 

 fect septa, often forming a filament. 



Climacosphenia, Orammatophora, Rhtibdonema, Tabellaria, Stria- 

 teUa. 



Family 8. Surirellese. Frustules alate, or carinate ; frequently 

 cuneate in front view and side view. 



Nilzxchia, SurirMt, Cymat -pleura. 



TRIBE III. CRYPTO-RAPEIDIE/E. 



Frustules cylindrical or angular ; frequently with processes, spines, 

 teeth, or awns ; and often coherent, forming a filament. 



Family 9. Chaetocereae. Frustules mostly hyaline and armed 

 with bristles or awns, and generally coherent. 



Rhizosolenia, Chcetoceros. 



Family 10. Melosirese. Frustules cylindrical, adhering and form- 

 ing a stout filament ; valves cylindrical, sometimes armed with spines. 



Melosira, Stephanopyxi*. 



Family 11. Biddulphieee. Frustules adherent, forming generally 

 a zigzag filament, attached by one or two processes. 



Istkmin, Terpsinoe, Biddulp^ia, Hemiaulus. 



Family 12. Eupodisceaa. Frustules not forming a filament ; 

 valves cylindrical, with ocelli ; often with radial ribs or furrows. 



Aulucus, Aulucodiscux, Evpodi*cus. 



Family 13. Heliopelfceae. Valves divided into compartments al- 

 ternately light and dark, often with marginal spines or teeth. 



Actinoptychui, Heliopelt i, Halionyx. 



Family 14. Asterolamprese. Valves circular (rarely angular) and 

 mostly hyaline, with linear, often bifurcating, rays. 



Actinodiacus, Mattogonia, Aslerolampra. 



Family 15. Coscinodisceae. Valves circular, generally with radi- 

 ating cellules, granules, or punctse ; sometimes with marginal or intra- 

 marginal spines or distinct ribs ; without distinct processes. 



Cydottlla, Actinocydu*, Stephatiodissus, Arachnoiducus, Co&cino- 

 discus. 



(b) Diatoms are very easily obtained for study ; it is only necessary 

 to scrape off a little of the slippery covering of submerged stones or 

 sticks to procure numerous specimens. They may be obtained also 

 from ordinary drinking water, allowing it to flow from a hydrant 

 through a filter of " Canton flannel " for an hour or so. Often appar- 



